This invention relates to ballast circuits for igniting and stabilizing the operation of one or more electric discharge lamps, and more particularly to a so-called hybrid ballast circuit having a high efficiency, improved starting characteristics and providing superior protection against the hazards of electrical shock.
One prior art hybrid ballast circuit arrangement is described in U.S. Pat. No. 3,997,814, issued Dec. 14, 1976 in the name of Makoto Toho. The basic operating circuit of the Toho device consists of the input supply voltage, a capacitor C, an inductor L and a discharge lamp connected in series circuit, and a semiconductor switch connected in shunt with the discharge lamp. This simplified form of the circuit is shown in FIG. 2 of the Toho patent and is described in column 2 of that patent. The semiconductor switch is actuated once in each half cycle of the A.C. supply voltage. This ballast circuit is compact in size and provides lamp ignition even with a supply voltage that is close to that of the lamp voltage.
A major disadvantage of the Toho system is that during the starting operation of a rapid start discharge tube the open circuit voltage applied across the tube electrodes may rise to such a high level as to cause the discharge tube to start instantaneously before the preheatable filament electrodes are heated to the proper operating temperature. It is well known that cold ignition of this type of discharge lamp is detrimental to the lamp life, i.e. the useful life of the discharge tube is reduced as a result of repeated ignitions thereof with insufficiently preheated filaments.
An improved version of the apparatus described in the Toho patent is presented in U.S. Pat. No. 4,253,043 which issued on Feb. 24, 1981 to H. M. J. Chermin et al. This patent discloses a hybrid ballast circuit provided with a voltage dependent resistor (VDR) connected in the gate control circuit of the semiconductor switch, e.g. a Triac, and arranged so that the Triac is triggered into conduction when the voltage applied to the discharge lamp, or lamps, reaches a predetermined level. The predetermined voltage level is chosen so as to prevent cold ignition of the lamps and thereby provides consistent rapid starting which is reliable and is not detrimental to the lamp life. As soon as the discharge lamps ignite, the VDR element reverts to its high impedance state so that it effectively has no further effect on the ballast circuit during operation of the lamps.
A second disadvantage of the ballast circuit described in the Toho patent is that in the event the discharge lamp does not start, but the lamp filaments are intact, the high starting current will continue to flow through the ballast circuit and will in time damage or destroy the ballast inductor and possibly other components of the apparatus. This problem is solved in the Chermin et al ballast apparatus by providing a positive temperature coefficient (PTC) resistor connected in series with the Triac switch. The PTC resistor is chosen and rated so that if the discharge lamp fails to ignite, the PTC resistor will be heated internally by the current, approximately 1.2 amps., and will shift from its low resistance state to its high resistance state after a predetermined period of time, for example, 30 seconds. A typical PTC resistor can switch from a low resistance value of 6 ohms, for example, to a resistance value of several thousand ohms when heated. It will thereby limit the current flow to a very low value and effectively shut down the system so as to prevent damage to any of the components.
It is a requirement of the U.S. National Electric Code that fluorescent fixtures used indoors be equipped with thermal protection. This has traditionally been accomplished by using bi-metal thermostats in the ballast. It is required that this thermal protection prevent the ballast from reaching excessive temperatures in the event of end-of-life faults of the ballast or in the event the ballast is installed in unacceptable conditions. The PTC element in this circuit also can be utilized to shut off the system in the event of such faults and/or improper application since it is sensitive to both current and temperature. The PTC resistor thus provides the thermal protection that present regulations require of a fluorescent ballast.
Although the ballast system disclosed by Chermin et al offers considerable improvement over the Toho apparatus, it too provides less than optimum results. An important limitation on both the Toho and Chermin et al apparatus is that they do not satisfy the safety requirements prevalent in the industry as to electric shock. In order to protect persons replacing a discharge lamp in a fixture from excessive electric shock, the Underwriter Laboratories specify certain maximum values of the RMS voltages present and the peak allowable voltage between a lamp socket and ground. The ballast circuits shown in the Toho and Chermin et al patents do not comply with this safety requirement thereby limiting their utility in a commercially acceptable ballast device.
A second possible area of improvement over the Chermin et al ballast relates to minimizing the power losses while avoiding instabilities of the system which manifest themselves as a flickering action during the warm-up or starting peiod of the lamp. This instability or flicker can be especially prominent when using the aforesaid ballast circuit with the 35 watt so-called energy saving type of discharge lamps which have a gas mixture containing krypton.
The lamp flicker occurs at a sub-harmonic of the AC supply frequency when starting such a discharge lamp. This instability or flicker may be of a violent nature under some circumstances and may be merely objectionable under other circumstances. The tendency towards instability of the particular discharge lamp is related to its temperature. If the lamps are started in a cool environment (for example 60.degree. F.) the instability will tend to persist until the lamps warm up sufficiently. If the lamps are prevented from warming, for example by a cold draft blowing across them, then the instability can persist. Energy saver lamps also exhibit this tendency with conventional ballast devices during start-up, but the instability usually disappears very quickly.
Our analysis of the flicker problem indicates that it is caused by a partial lamp rectification that takes place during lamp start-up and which tends to allow more current to flow in one direction than in the other. This flicker usually occurs after a lamp ignites because the lamp does not reignite sufficiently during one-half cycle of the A.C. supply voltage. As a result an imbalance occurs in the system to produce an excessive build-up voltage on the ballast capacitor during one-half cycle and a very substantial lamp current then flows in the next successive half cycle. The voltage across the capacitor increases in the one direction until it reaches a level where, together with the A.C. supply voltage, it causes the lamps to conduct, thereby producing a very bright flash of light. This action repeats and results in the objectionable flicker referred to above.
Tests have shown that lamp flicker can be reduced and stable lamp operation provided by increasing the size of the inductance in series with the capacitor and the lamp since the larger inductance provides an additional smoothing of the lamp current and thus prevents or at least minimizes any flickering. This solution, although it provides more stable lamp operation during the start-up period, is objectionable in that it requires an inductor of greater size and cost and which causes greater energy losses.
A ballast device having certain improved operating characteristics over the ballast disclosed in the Chermin et al patent is described in U.S. application Ser. No. 207,321, filed Nov. 17, 1980, now U.S. Pat. No. 4,380,719